The present invention refers to a two-piece connecting rod and to a method for assembling a two-piece connecting rod in a reciprocating hermetic compressor of the type used in small refrigerating machines, such as refrigerators, freezers, drinking fountains, etc.
The reciprocating compressors used in refrigerating systems have a connecting rod, which is mounted, at one side, to an eccentric of a crankshaft supported on a cylinder block and, at the other side, to a piston reciprocating inside a cylinder, with a stroke orthogonal in relation to the eccentric axis, between a lower dead point condition and an upper dead point condition, which are respectively defined by a maximum and by a minimum linear spacing between the piston top and a valve plate, which is mounted to a cylinder end and which defines an inner face of a cylinder head assembly defining suction and discharge chambers of the compressor.
In these constructions, in the upper dead point condition, the piston should have a certain spacing from an inner face of the valve plate of the cylinder head, defining, in this region, a certain minimum dead volume, which is previously determined in order to avoid, during normal operation of the compressor, the occurrence of impacts of the piston against the cylinder head.
In order to avoid the performance of the compressor to be impaired, the dead volume produced in the region adjacent to the cylinder head should have a minimum value, which also avoids loss of compressor yield.
As a function of the reduced size of the components and of the exiguous space for mounting the piston-connecting rod-eccentric pin assembly, different constructive alternatives for the connecting rod have arisen, aiming at facilitating to mount this assembly to the compressor, among which the ones using a two-piece connecting rod, due to the facility to mount the latter to the eccentric and piston parts.
In the known constructions of a two-piece connecting rod, the mutual fitting between the connecting rod portions, each to be mounted to a respective part defined by the eccentric or the piston, is achieved so that the connecting rod presents, with the fixation of said connecting rod portions, its designed length, which is previously established as a function of a nominal design distance between the eccentric axis and a plane coinciding with the outer face of the cylinder top, in the upper dead point condition.
As a function of this previous length determination and of the dimensional variations existing in the compressor components and which may modify the relative position between the eccentric and piston parts, the assembly of the connecting rod to said parts usually determines a different length from that optimum desired length.
In order to facilitate the determination of the dead volume between the top of the piston head and a plane containing the inner face of the cylinder head, in the constructions in which the valve plate is affixed to the cylinder after mounting the piston thereto, the connecting rod is dimensioned so that the top of the piston head be substantially coplanar to the plane containing the cylinder top, in the upper dead point condition. The determination of the desired dead volume is obtained by combining gaskets of distinct thicknesses provided between the cylinder top and the valve plate.
For the constructions in which the assembly of the piston to the cylinder occurs after attaching the valve plate to the cylinder, the dimensioning of the dead volume depends upon the dimensional characteristics of the parts involved in the assembly (piston, connecting rod, cylinder, etc.), the control of said dead volume being a function of the perfect dimensioning of said parts.
The high precision required for the components involved in mounting the piston to the eccentric leads to high costs for obtaining these components. Moreover, the known solutions for a two-piece connecting rod have many components and are difficult to assemble.
Thus, it is the object of the present invention to provide a two-piece connecting rod and a method for assembling a two-piece connecting rod which permits the piston/connecting rod/eccentric assembly to be easily and quickly mounted, without requiring high dimensional precision for each of the involved parts, but which permits the dead volume in the cylinder to be controlled more precisely, independently of the type of the construction and of the method for mounting said assembly.
This object of the invention is achieved by a two-piece connecting rod for a reciprocating compressor, comprising a first connecting rod portion to be mounted to the eccentric of a crankshaft of the compressor and a second connecting rod portion to be articulated to a piston reciprocating inside a cylinder with an end closed by a cylinder head, said connecting rod comprising: an engaging portion, provided in one of the parts defined by the first and the second connecting rod portions; a guide portion, provided in the other of said parts, for coupling with the engaging portion, said coupling allowing the mutual relative displacement between said parts towards the axis of the connecting rod; and a fixation means, applied to the coupling region between the engaging portion and the guide portion, the first connecting rod portion being mounted to the eccentric and the second connecting rod portion being mounted to the piston lodged in the cylinder, in order to lock said portions against relative displacements in a position which results in a predetermined axial distance between the top face of the piston, in the upper dead point condition, and the inner face of the cylinder head of the cylinder.
This connecting rod is obtained by a method including the steps of:
axe2x80x94mounting the first connecting rod portion to the eccentric of the crankshaft;
bxe2x80x94coupling, with a certain degree of mutual mechanical interference, the engaging portion with the guide portion;
cxe2x80x94sliding, axially towards the axis of the connecting rod, the engaging portion in relation to the guide portion, until the top face of the piston mounted inside the cylinder remains, in the upper dead point condition of the eccentric, at a predetermined axial distance from the inner face of the cylinder head of the cylinder;
dxe2x80x94retaining said parts defined by the engaging portion and guide portion to each other, against involuntary relative axial displacements, by means of said mutual mechanical interference; and
exe2x80x94mutually affixing the parts defined by the engaging portion and guide portion through a fixation means applied to the mutual coupling region between said connecting rod portions.